The pharmacodynamic concept of 'spare receptors' (receptor reserve) explains which clinical phenomenon BEST?

• A Full agonist maximum effect can be achieved at much less than 100% receptor occupancy, allowing a margin of safety ✓
• B Partial agonists always achieve less than 50% of full agonist effect regardless of dose
• C Competitive antagonists shift the dose-response curve to the left in the presence of excess agonist
• D Tolerance develops faster in tissues with high receptor density

Explanation

Spare receptors (receptor reserve) refers to the phenomenon where maximal tissue response can be achieved when only a fraction (sometimes as few as 1–5%) of available receptors are occupied by a full agonist. This occurs because receptor activation initiates an amplified intracellular signalling cascade. Clinical implication: irreversible antagonists (e.g., phenoxybenzamine) must block a substantial proportion of receptors before reducing the agonist maximum effect, because unblocked spare receptors can still generate a full response. This explains why low doses of agonist produce sub-maximal effect — excess unoccupied receptors can be recruited.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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